To prove it or disprove it, would require complete DNA sequences, or at least lengthy sequences, of both living descendent species as well as the DNA sequences from assumed common ancestor species.
No, all it would require would be finding something that, eg., uses a different genetic code. Or has bones made of cast iron.
Or was an intermediate form between lineages that are not related in that way. For example, something with feathers *identical* (in biochemical detail) to bird feathers, but with the skeletal features of a mammal or amphibian.
Granted, some bizarre finds like these would *probably not* be considered evidence against standard biology, but would probably be taken as evidence that the one creature that seems to violate it is not native to the Earth.
However, if a large percentage of Earth's life were to be like this, it would disprove evolution theory.
Another possible disproof would be if the phylogeny derived from anatomy, biogeography, behavior, etc, (pre-1950s biology) were to differ greatly from that inferred from molecular studies of proteins and dna, or if the biochemical data didn't conform to any sort of tree structure. Of course, in the real world, the biochem data complements and provides supporting detail to the already-known taxonomy.
Now here's one for you: State the atomic theory of matter in a succint, disprovable form.
I won't try that. The "atomic theory" of matter was effectively disproved once they started busting out subatomic particles in the original "atom smashers". And I sure don't remember the original atomic theory since I don't think they have been teaching that since...heck, I don't know. The '40s?
The problem is, the DNA evidence just shows that all life on earth is related. It doesn't require descent from common ancestors. Clearly the concept of intelligent design also fits that bill. And intelligent design doesn't necessarily mean God with a big G. Could be aliens. Or devas. Or angels. Or Chuthulu. Or whatever.
The fundamental issue is that evolutionary theory, as stated (and vigorously challenged in various posts by me), seems to merely hold up in two cases that can be experimentally demonstrated, which are: (a) plants, and (b) within a species, that is, breeds. The evidence for species differentiation appears to be inferred. Having a CS background, I know the jokes and understand the dangers that occur when you attempt to prove your theory and end up saying "now, by induction therefore...".
Joke:
The CS prof says: 1 is prime, and 3 is prime, and 5 is prime, and 7 is prime, therefore by induction all odd integers are prime.
(And the physics prof says: 1 is prime, and 3 is prime, and 5 is prime, and 7 is prime, and 9 is, uh, experimental error, and 11 is prime, and 13 is prime -- therefore all odd integers are prime within the bounds of experimental error.)
So whenever I see the bio types in their experiments say: well, it would take too many generations, and too long, and too much money to actually prove this, so therefore we infer that species differentiation would eventually result, therefore evolutionary theory has been proved, QED...I am not willing to accept their inference as evidence.